Selective staging hydrocracking

ABSTRACT

This invention is directed to a high conversion hydrocracking (HCR) unit to produce premium middle distillate fuel. Unconverted oil which is low in sulfur is fed to a Fluid Catalytic Cracking (FCC) unit. The process results in reduced hydrogen consumption and optimum reactor capacity. Feed is hydrotreated and separated into liquid and vapor streams. The vapor stream is passed to further processing and light ends recovery. The liquid stream is passed to a vacuum distillation column, where it is separated into at least three streams, the first stream comprising low boiling products and light ends, a second, higher boiling stream comprising the feed to a second hydroprocessing zone and third stream comprising unconverted oil. The second stream is passed to the second hydroprocessing zone, producing effluents which boil in the distillate range. The second hydroprocessing zone is generally a fuels hydrocracking unit.

FIELD OF THE INVENTION

This invention is directed to a high conversion hydrocracking (HCR) unit to produce premium middle distillate fuel. Unconverted oil which is low in sulfur is fed to a Fluid Catalytic Cracking (FCC) unit. The process results in reduced hydrogen consumption and optimum reactor capacity.

BACKGROUND OF THE INVENTION

In the refining of crude oil, vacuum gas oil hydrotreaters and hydrocrackers are employed to remove impurities such as sulfur, nitrogen and metals from the feed. Typically, the middle distillate boiling material (boiling in the range from 250° F. through 735° F.) from VGO hydrotreating or moderate severity hydrocrackers does not meet the smoke point, the cetane number or the aromatic specification required.

Removal of these impurities in subsequent hydroprocessing stages (often known as upgrading), creates more valuable middle distillate products. Hydroprocessing technology (which encompasses hydrotreating, hydrocracking and hydrodewaxing processes) aims to increase the value of the crude oil by fundamentally rearranging molecules. The end products are also made more environmentally friendly.

In most cases, this middle distillate is separately upgraded by a middle distillate hydrotreater or, alternatively, the middle distillate is blended into the general fuel oil pool or used as home heating oil. Recently hydroprocessing schemes have been developed which permit the middle distillate to be hydrotreated in the same high pressure loop as the vacuum gas oil hydrotreating reactor or the moderate severity hydrocracking reactor. The investment cost saving and/or utilities saving are significant since a separate middle distillate hydrotreater is not required.

There are U.S. patents which are directed to multistage hydroprocessing within a single high pressure hydrogen loop. In U.S. Pat. No. 6,797,154, high conversion of heavy gas oils and the production of high quality middle distillate products are possible in a single high-pressure loop with reaction stages operating at different pressure and conversion levels. The flexibility offered is great and allows the refiner to avoid decrease in product quality while at the same time minimizing capital cost. Feeds with varying boiling ranges are introduced at different sections of the process, thereby minimizing the consumption of hydrogen and reducing capital investment.

U.S. Pat. No. 6,787,025 also discloses multi-stage hydroprocessing for the production of middle distillates. A major benefit of this invention is the potential for simultaneously upgrading difficult cracked stocks such as Light Cycle Oil, Light Coker Gas Oil and Visbroken Gas Oil or Straight-Run Atmospheric Gas Oils utilizing the high-pressure environment required for hydrocracking.

U.S. Pat. No. 7,238,277 provides very high to total conversion of heavy oils to products in a single high-pressure loop, using multiple reaction stages. The second stage or subsequent stages may be a combination of co-current and counter-current operation. The benefits of this invention include conversion of feed to useful products at reduced operating pressures using lower catalyst volumes. Lower hydrogen consumption also results. A minimal amount of equipment is employed. Utility consumption is also minimized.

U.S. Publication 20050103682 relates to a multi-stage process for hydroprocessing gas oils. Preferably, each stage possesses at least one hydrocracking zone. The second stage and any subsequent stages possess an environment having a low heteroatom content. Light products, such as naphtha, kerosene and diesel, may be recycled from fractionation (along with light products from other sources) to the second stage (or a subsequent stage) in order to produce a larger yield of lighter products, such as gas and naphtha. Pressure in the zone or zones subsequent to the initial zone is from 500 to 1000 psig lower than the pressure in the initial zone, in order to provide cost savings and minimize overcracking.

A waxy vacuum column has traditionally been used downstream of fractionator to segregate the feed to the isomerization dewaxing unit from a slip stream from the vacuum column which is recycled to second stage hydrocracker. Generally the first stage is run at sufficiently high conversion to ensure that the unconverted oil is suitable as second stage hydrocracking or isomerization dewaxing feed.

SUMMARY OF THE INVENTION

Selective Staging Hydrocracking (SSH) is employed to process high severity feed which contains high boiling straight run vacuum gas oil (VGO). This VGO is obtained from difficult crude sources which are blended with a high percentage of Heavy Coker Gas Oil (high polycyclic aromatics, nitrogen content in the range of 5000 ppm and above). In this scheme, the first stage hydrocracker operation and severity is optimized (lowered) to meet the FCC feed quality requirements, which are much less stringent than the requirements for the quality of the unconverted oil which feeds a second stage hydrocracker or an isomerization dewaxing unit. In addition, in the SSH scheme, 100% of second stage feed is the middle cut, or heart cut side draw from the vacuum column.

This unique combination of selective conversions in the first stage and second stage, using a vacuum column to process very difficult feed containing a high percentage of heavy cracked stock to produce premium grade jet and diesel fuel, is first of a kind process application. Total reactor volume is about 30% lower compared to previous schemes processing similar feed to achieve similar performance targets.

This invention is directed to a high conversion hydrocracking (HCR) unit to produce premium middle distillate fuel. Unconverted oil which is low in sulfur is fed to a Fluid Catalytic Cracking (FCC) unit. The process results in reduced hydrogen consumption and optimum reactor capacity.

The patents disclosed above do not address the following approaches to obtaining a high yield of premium quality jet and diesel products:

-   -   1. Hydrotreating of fresh feed and subsequent passing of         unconverted oil to a vacuum column. A side draw from the vacuum         column is sent to a hydrocracking reactor which processes clean         feeds (feeds comprising few heteroatoms) and the bottoms are         sent to an FCC unit. The amount of bottoms sent to FCC is         dependent upon desired overall conversion.     -   2. A process having flexibility to adjust the severity of the         hydrotreating reactor to handle relatively high end point feed         as well as feed variations from upstream adjustments, while         maintaining a steady feed from a vacuum column to the clean         hydrocracking reactor for full conversion to ultra high quality         distillate products.     -   3. Means of avoiding undesirable over-saturation of the         unconverted oil (UCO) from the hydrocracking unit feeding the         FCC unit, resulting in a significant reduction in hydrogen         consumption     -   4. Reduced reactor costs, due to the fact that total reactor         volume is about 30% lower compared to previous schemes         processing similar feed to achieve similar performance targets.

A new, innovative hydrocracking process scheme has been developed that provides maximum flexibility in terms of feed quality variations and desired overall conversion to distillate products at minimum investment cost. Fresh feed oil is first processed in the ISOTREATING reactor containing hydrotreating and some hydrocracking catalyst to achieve near complete demetallation, hydrodenitrifrication, hydrodesulfurization and some aromatic saturation to meet the FCC feed specifications.

The reactor effluents are send to recycle gas separation, product fractionation and lights ends recovery sections. In this new process scheme, the atmospheric fractionator bottom stream containing heavier than diesel boiling material is sent to a vacuum column, a first time application in fuels HDC, to separate the second stage fed from the UCO. A heart-cut side draw is taken from the vacuum column and sent to a clean Second-Stage ISOCRACKING reactor for complete conversion to high quality distillate products. The unconverted oil from Vacuum column bottom is an excellent quality FCC feed.

The following are some of the unique advantages of this new scheme with the new second stage feed vacuum column:

-   -   1. Capability to adjust the first stage reactor severity to         produce on-specification FCC feed with varations in hydrocracker         feedstocks and flexibility to vary the second stage feed rate         from the new vacuum column to achieve desired overall         conversion.     -   2. Ability to maintain a sharp cut, steady feed flow to the         clean second stage reactor for full conversion to high quality         mid-distillate products.     -   3. Higher fraction of premium quality distillate products from         the clean second stage with lower first stage conversion         compared to the scheme without second stage feed vacuum column.     -   4. Selective hydrogenation to minimize overall hydrogen         consumption.     -   5. Selective staging requires lower reactor volume for similar         catalyst system and performance targets. With the second stage         feed vacuum column, the first stage reactor can be operated at         much lower severity as compared to existing industry practice to         send fractionator bottoms to clean second stage hydrocracking         reactor. Also, the heart-cut side-draw taken from the vacuum         column constitutes a relatively easier to process feed to the         clean second stage reactor. Total reactor weight for the         combined first and second stage will be about 30% lower in         selective staging hydrocracking.     -   6. Flexibility to draw high quality waxy lube base oil form the         vacuum column

BRIEF DESCRIPTION OF THE FIGURE

The FIGURE illustrates the flow scheme of the current invention.

DETAILED DESCRIPTION OF THE INVENTION Feeds

A wide variety of hydrocarbon feeds may be used in the instant invention. Typical feedstocks include any heavy or synthetic oil fraction or process stream having a boiling point above 392° F. (200° C.). Such feedstocks include vacuum gas oils (VGO), heavy coker gas oil (HCGO), heavy atmospheric gas oil (AGO), light coker gas oil (LCGO), visbreaker gas oil (VBGO), demetallized oils (DMO), deasphalted oil (DAO), Fischer-Tropsch streams, Light Cycle Oil, Light Cycle Gas Oil and other FCC product streams.

Products

The process of this invention is especially useful in the production of middle distillate fractions boiling in the range of about 250-700° F. (121-371° C.). A middle distillate fraction is defined as having an approximate boiling range from about 250 to 700° F. At least 75 vol %, preferably 85 vol % of the components of the middle distillate have a normal boiling point of greater than 250° F. At least about 75 vol %, preferably 85 vol % of the components of the middle distillate have a normal boiling point of less than 700° F. The term “middle distillate” includes the diesel, jet fuel and kerosene boiling range fractions. The kerosene or jet fuel boiling point range refers to the range between 280 and 525° F. (138-274° C). The term “diesel boiling range” refers to hydrocarbons boiling in the range from 250 to 700° F. (121-371° C.).

Gasoline or naphtha may also be produced in the process of this invention. Gasoline or naphtha normally boils in the range below 400° F. (204° C.), or C₅ to 400° F. Boiling ranges of various product fractions recovered in any particular refinery will vary with such factors as the characteristics of the crude oil source, local refinery markets and product prices.

Conditions

“Hydroprocessing conditions” is a general term which refers primarily in this application to hydrocracking or hydrotreating.

Hydrotreating conditions include a reaction temperature between 400° F.-950° F. (204° C.-482° C.), preferably 600° F.-850° F. (315° C.-464° C.); a pressure between 500 to 5000 psig (pounds per square inch gauge) (3.5-34.6 MPa), preferably 1000 to 3000 psig (7.0-20.8 MPa): a feed rate (LHSV) of 0.3 hr-1 to 20 hr-1 (v/v) preferably from 0.5 to 4.0; and overall hydrogen consumption 300 to 2000 SCF per barrel of liquid hydrocarbon feed (63.4-356 m³/m³ feed).

Typical hydrocracking conditions include a reaction temperature of from 400° F.-950° F. (204° C.-510° C.), preferably 650° F.-850° F. (315° C.-454° C.). Reaction pressure ranges from 500 to 5000 psig (3.5-4.5 MPa), preferably 1000-3000 psig (7.0-20.8 MPa). LHSV ranges from 0.1 to 15 hr-1 (v/v), preferably 0.5 to 5.0 hr-1. Hydrogen consumption ranges from 500 to 2500 SCF per barrel of liquid hydrocarbon feed (89.1-445 m³H₂/m³ feed).

Catalyst

A hydroprocessing zone may contain only one catalyst, or several catalysts in combination.

The hydrocracking catalyst generally comprises a cracking component, a hydrogenation component and a binder. Such catalysts are well known in the art. The cracking component may include an amorphous silica/alumina phase and/or a zeolite, such as a Y-type or USY zeolite. Catalysts having high cracking activity often employ REX, REY and USY zeolites. The binder is generally silica or alumina. The hydrogenation component will be a Group VI, Group VII, or Group VIII metal or oxides or sulfides thereof, preferably one or more of molybdenum, tungsten, cobalt, or nickel, or the sulfides or oxides thereof. If present in the catalyst, these hydrogenation components generally make up from about 5% to about 40% by weight of the catalyst. Alternatively, platinum group metals, especially platinum and/or palladium, may be present as the hydrogenation component, either alone or in combination with the base metal hydrogenation components molybdenum, tungsten, cobalt, or nickel. If present, the platinum group metals will generally make up from about 0.1% to about 2% by weight of the catalyst.

Hydrotreating catalyst is typically a composite of a Group VI metal or compound thereof, and a Group VIII metal or compound thereof supported on a porous refractory base such as alumina. Examples of hydrotreating catalysts are alumina supported cobalt-molybdenum, nickel sulfide, nickel-tungsten, cobalt-tungsten and nickel-molybdenum. Typically, such hydrotreating catalysts are presulfided.

In some cases, high activity hydrotreating catalyst suitable for high levels of hydrogenation, is employed. Such catalysts have high surface areas (greater than 140 m.sup.2/gm) and high densities (0.7-0.95 gm/cc). The high surface area increases reaction rates due to generally increased dispersion of the active components. Higher density catalysts allow one to load a larger amount of active metals and promoter per reactor volume, a factor which is commercially important. Since deposits of coke are thought to cause the majority of the catalyst deactivation, the catalyst pore volume should be maintained at a modest level (0.4-0.6). A high activity catalyst is at times desired in order to reduce the required operating temperatures. High temperatures lead to increased coking.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Please refer to the FIGURE:

In this process scheme, the feed which is hydrotreater reactor effluent (Stream 7) is passed to furnace 15 and proceeds through line 8 to the base of the stripping and rectification section of the vacuum distillation column 20. Steam enters below the bottom bed of the vacuum distillation column through stream 9. The feed is separated in the distillation column into streams with different boiling ranges. The lightest materials proceed to the vacuum system through stream 11. Stream 12 is circulated by reflux pump 65 through stream 13 to cooler 45 to a lighter level tray through stream 14. Recirculation permits further processing of streams for more complete separation and greater flexibility of product specification.

Stream 16 is the middle, or “heart cut” side draw which is transferred via pump 75 from the vacuum column, and as stream 17, passes to storage drum 30.

The unconverted oil, stream 23, from the bottom of vacuum column 20 is an excellent quality FCC feed. Stream 23 exits pump 85 as stream 24. Vacuum distillation is typically used to separate the higher boiling material, such as the lubricating base oil fractions, into different boiling range cuts. Fractionating the lubricating base oil into different boiling range cuts enables the lubricating base oil manufacturing plant to produce more than one grade, or viscosity, of lubricating base oil. Stream 24 is heated, via exchanger 35 and proceeds to the Fluid Catalytic Cracking Unit as stream 26. Further saturation of the FCC feed is thus avoided.

The heart cut stream exits storage drum 30 through stream 18 and is pumped, by means of pump 55, to stream 19. Stream 19 is heated in exchanger 25 prior, and then exits the exchanger as stream 21. Stream 21 is heated in furnace 5 prior to entering hydrocracking reactor 10 for further aromatic saturation. Hydrogen is depicted as entering between beds as streams 2 and 3. Unconverted oil exits the reactor via stream 4. The unconverted oil is cooled in exchanger 25 and exits as stream 6.

EXAMPLE

The following table highlights the advantages of the process scheme of this invention over a conventional process scheme for a 45,000 BPOD (barrel per operating day) hydrocracking unit:

Conventional Process Scheme New Process Scheme Process Scheme Two Stage Recycle Selective Staging Hydrocracking Hydrocracking Fresh Feed Rate, BPSD 45,000 45,000 Overall Conversion 70% to Diesel and 70% to Diesel and Lighter Products Lighter Products Overall LGSV, 1/hr 0.54 0.70 Product Yield Base Similar to Base case Jet and Diesel Quality Base Jet Smoke Point: Base + 1 mm ULSD Cetane Index: Base + 2 Chemical H₂ Consumption Base Base - 180 SCFB Reactor Weight Base Base - 450 Metric Ton Total Equipment Cost Base Base - 7.0 US$MM

Comparison of the two schemes indicates that, in the scheme of the instant invention, there is greater space velocity, meaning lower residence time in the reactors. The jet fuel shows an improvement with higher smoke point and the diesel shows an improved cetane number. Less hydrogen is consumed, smaller reactors may be employed and total equipment cost is about $7 million less. 

1. A method for hydroprocessing a hydrocarbon feedstock, which provides maximum flexibility in terms of feed quality variations and desired overall conversion to distillate products at minimum investment cost, said method employing multiple hydroprocessing zones within a single reaction loop, each zone having one or more catalyst beds, comprising the following steps: (a) passing a hydrocarbonaceous feedstock to a first hydroprocessing zone having one or more beds containing hydroprocessing catalyst, the hydroprocessing zone being maintained at hydroprocessing conditions, wherein the feedstock is contacted with catalyst and hydrogen; (b) passing the effluent of step (a) directly to a separator, wherein the effluent is separated to produce a vapor stream comprising hydrogen, hydrogen sulfide, ammonia, and hydrocarbonaceous compounds boiling at a temperature below the boiling range of a diesel stream, and a liquid stream comprising hydrocarbonaceous compounds boiling at a temperature above the range of a diesel stream; (c) passing the vapor stream of step (b) to further processing and light ends recovery, and passing the liquid stream of step (b) to a vacuum distillation column, where it is separated into at least three streams, the first stream comprising low boiling products and light ends, a second, higher boiling stream comprising the feed to a second hydroprocessing zone and third stream comprising unconverted oil; (d) passing the second stream of step (c) to the second hydroprocessing zone, producing effluents which boil in the distillate range.
 2. The process of claim 1, wherein the second hydroprocessing zone of step (d) is a fuels hydrocracker.
 3. The process of claim 1, wherein production of specific products may be optimized in the distillation column by recycle between trays.
 4. The process of claim 1, wherein at least a portion of the stripped unconverted oil of step (c) is passed to a fluid catalytic cracking unit as feed.
 5. The process of claim 1, wherein the second hydroprocessing zone contains at least one bed of hydroprocessing catalyst suitable for aromatic saturation and ring opening.
 6. The process of claim 1, wherein the second stream is contacted under hydroprocessing conditions with the hydroprocessing catalyst, in the presence of hydrogen to produce middle distillate products.
 7. The process of claim 1, wherein the hydroprocessing conditions of step (a) comprise a reaction temperature of from 400° F.-950° F. (204° C.-510° C.), a reaction pressure in the range from 500 to 5000 psig (3.5-34.5 MPa), an LHSV in the range from 0.1 to 15 hr-1 (v/v), and hydrogen consumption in the range from 500 to 2500 scf per barrel of liquid hydrocarbon feed (89.1-445 m³ H₂/m³ feed).
 8. The process of claim 7, wherein the hydroprocessing conditions of step 1(a) comprise a temperature in the range from 650° F.-850° F. (343° C.-454° C.), reaction pressure in the range from 1500-3500 psig (10.4-24.2 MPa), LHSV in the range from 0.25 to 2.5 hr-1, and hydrogen consumption in the range from 500 to 2500 scf per barrel of liquid hydrocarbon feed (89.1-445 m³ H₂/m³ feed).
 9. The process of claim 1, wherein the hydroprocessing conditions of step 1(c) comprise a reaction temperature of from 400° F.-950° F. (204° C.-510° C.), a reaction pressure in the range from 500 to 5000 psig (3.5-34.5 MPa), an LHSV in the range from 0.1 to 15 hr-1 (v/v), and hydrogen consumption in the range from 500 to 2500 scf per barrel of liquid hydrocarbon feed (89.1-445 m³ H₂/m³ feed).
 10. The process of claim 1, wherein the feed to step 1(a) comprises hydrocarbons boiling in the range from 500° F. to 1500° F.
 11. The process of claim 1, wherein the feed is selected from the group consisting of vacuum gas oil, heavy atmospheric gas oil, delayed coker gas oil, visbreaker gas oil, FCC light cycle oil, and deasphalted oil.
 12. The process of claim 1, wherein the hydroprocessing catalyst comprises both a cracking component and a hydrogenation component, wherein the cracking component may be amorphous or zeolitic.
 13. The process of claim 12, wherein the hydrogenation component is selected from the group consisting of Ni, Mo, W, Pt and Pd or combinations thereof.
 14. The process of claim 12, wherein the zeolitic component is selected from the group consisting of Y, USY, REX, and REY zeolites.
 15. The process of claim 1, wherein the hydrotreating occurs in the first reaction zone and hydrocracking occurs in the second reaction zone. 